This invention relates generally to laser beam projectors or telescopes, and, more particularly, to a light spot position sensor for a wavefront sampling system.
Laser beams have a number of remarkable properties. Because of their spatial coherence, they have an extremely small divergence and are therefore highly directional. A laser beam because it possesses space coherence, can be focused to form a spot whose diameter is of the order of one wavelength of the laser light itself. Enormous power densities are thus obtainable. Accordingly, system applications of lasers are useful for communication in space, on earth and undersea, as well as in surveillance and weapon systems.
In may laser systems it is desirable to concentrate the projected laser energy into a small area in the object plane. In order to accomplish this end, large optics in the projector system are required. To achieve the high performance theoretically possible with such large optics, other parts of the projection system must also perform at comparable levels of high accuracy. For example, at diameters of 4.3 meters with .lambda.=2.7.mu., the radius of the Airy disc is only 0.77 .mu.R. To deposit energy upon a given target area of 3/4 of the ideal rate, the boresight error can be only about 0.2 .mu.R even assuming a perfect projected beam. On the other hand, if we assume a perfect boresight, the projected wavefront error allowable to achieve the 3/4 maximum deposition rate is only .lambda./13 rms.
To assure this high accuracy in the projected beam, a wavefront sampling system should incorporate therein the following characteristics:
1. It should have a negligible insertion loss;
2. It should provide a signal to the wavefront sensors which is sufficiently and uniformly attenuated;
3. It should sample over the entire aperture;
4. It should give a measurement of the projected wavefront phase accuracy after the wavefront has left the last optical surface;
5. It should be independent of those factors which do not affect energy density on the target (that is, laser wavelength changes, etc.);
6. It should measure net projected wavefront tilt (boresight) as well as relative wavefront inaccuracies; and
7. The wavefront analysis method should be capable of sufficient accuracy and signal to noise for general application.
The wavefront sampling system set forth in U.S. patent application Ser. No. 922,062 filed July 7, 1978 by the same inventor as the instant invention is capable of meeting the above operational criteria. However, the problem arises within the wavefront sampling system as how to accurately measure the relative spot positions of the generated beams at high bandwidths.